There is a pressing need for new systems for the destruction of organic hazardous waste containing toxic components. Such toxic components contain very harmful heteroatoms of halogens, nitrogen, phosphorous, sulfur and metals. The toxic components are found in typical hazardous wastes including all Freons, chlorinated solvents, i.e. those used in dry cleaning; halogen-based plastics, i.e. polyvinyl chloride (PVC) and the like; PCB-dielectric oils for transformers and capacitors; chemical and biological warfare agents; and all pesticides, herbicides, fumacides, bactericides, fungitides, and similar "cides".
Hazardous wastes are very difficult to dispose in landfills because they are biologically stable and tend to leach into the ground water. The use of incinerators for burning hazardous waste is becoming an environmentally unacceptable option for entities faced with its destruction. The destruction of hazardous wastes by incineration is very expensive because of the formation of toxic and corrosive acid gases which attack common refractory, mortars and common alloys and other metals used in incinerators. Incinerators that meet current air pollution laws and can be efficiently operated tend to be relatively large. As a consequence, such incinerators may not be economically feasible for placement at facilities where toxic waste is generated. Moreover, incinerators are often difficult to control and create strong community and political ill-feeling. Perhaps more importantly, however, the incineration process may produce other toxic products which are themselves undesirable and which are difficult or impossible to eliminate.
In U.S. Pat. No. 4,874,587, issued Aug. 26, 1987 and assigned to the assignee of the present invention, a process and apparatus are described for hazardous waste detoxification which represent a significant improvement over incinerators. In the aforesaid patent, a reactor is described in which toxic destruction levels of 99.99% or more are achieved. The aforesaid process and apparatus are operated without air or free oxygen reaction and produce an effluent gas which is primarily comprised of carbon dioxide, hydrogen, carbon monoxide and water. The process and apparatus described in the aforesaid patent has been classified by the United States Environmental Protection Agency as "infrared heater" as differentiated from "incineration". Federal Register No. 57, Aug. 25, 1992, pp. 38558-38564, EPA memorandum Sep. 30, 1991 from Sylvia Lowrence, Dir., Solid Waste to Allyn M Davis, Dir., Region 6.
A steam-reforming detoxification reactor operates to react a gaseous stream of toxic material with water in excess of the stoichiometric amount necessary to react with substantially all of the organic compounds in the stream of toxic waste. This reaction is carried out at a temperature in excess of about 1000.degree. C. (1832.degree. F.) and results in an effluent gas stream of high temperature comprised primarily of carbon dioxide, water, and hydrogen but also containing low levels of carbon monoxide. The latter can be readily converted later to carbon dioxide by catalytic oxidation.
Since the principal reaction in a steam-reforming detoxification reactor occurs in the gas phase, the processing of waste where the waste material can be relatively easily gasified is fairly straightforward. For example, a system for vaporizing and gasifying toxic waste for feeding to a steam-reforming detoxification reactor wherein the toxic waste is liquid contained in a metal drum is shown and described in U.S. Pat. No. 4,863,702, issued on Sep. 5, 1989 and assigned to the assignee of the present invention.
The use of steam-reforming systems for dehalogenation presents special problems since the basic chemistry involved is not well known. The present state of steam-dehalogenation chemistry can be illustrated by the pyrolysis and oxidative pyrolysis of methyl chloride in steam; see R. Yildirim and S. M. Senkan, "Pyrolysis and Oxidative Pyrolysis of Methyl Chloride in Steam", Ind. Eng. Chem. Res. 32, pages 438-444 (1993). The authors conclude that incineration in the presence of steam plus small quantities of oxygen are beneficial to eliminate soot and tars that are common in the pyrolysis of chlorinated hydrocarbons. The steam inhibits the formation of coke to a greater extent than in pyrolysis with the use of inert gas carriers such as argon and nitrogen; see A. Granada, S. M. Karra, S. M. Senkan, "Conversion of CH.sub.4 into C.sub.2 H.sub.2 and C.sub.2 H.sub.4 by the Chlorine-Catalyzed Oxidative Pyrolysis (CCOP) Process", Ind. Eng. Chem. Res. 26, page 1901 (1987) and S. M. Senkan, R. Yildirim, D. Palke, G. E. Lewis, "Conversion of Methane to Ethylene and Acetylene by Chlorine-Catalyzed Oxidative Pyrolysis Process", in Novel Production Methods for Ethylene, Light Hydrocarbons and Aromatics by L. F. Albright, B. L. Crynes, B. L. Nowak, and S. Eds, published by Marcel Dekker, New York, N.Y. (1992). The formation of such particulate matter as coke acts as a precursor or catalyst for the further formation of soot particles and eventually leads to the formation of tars and other harmful deposits. The use of incinerators forms halogen-containing dioxins (one of the most toxic compounds) and dibenzofurans in the post-combustion regions of the equipment where soot and particulate matter exist in excess. The formation of these two harmful heteroatom-substituted organic compounds occur together and the presence of one compound enhances the formation of the other compound. It is the condensation of the heavy polyaromatic hydrocarbons on these soot particles that create the environmental and human health hazardous conditions because these dioxin-laden particles are fine enough to be respirable and ingestible by humans and animals and also to fall onto edible surface crops. The latter is a problem common to dioxin, combustion formed soot, and halogens.
From a review of the published literature on the subject, only steam-pyrolysis removal of single-bonded chlorine is covered. The steam-reforming detoxifier disclosed and claimed in U.S. Pat. No. 4,874,587 handles multiple chlorinated and fluorinated hydrocarbons and chlorinated aromatics. Such complex molecules are dehalogenated in several steps, depending on the halogen-carbon bond energies. Even in the case of the relatively simple multiple halogenated aromatics such as dichlorobenzene, the first chlorine is removed easier than the second chlorine. This is probably due to the increase in the population of .pi.-orbitals around and strengthening of the last chlorine bonded to the ring. The treatment of such poly-halogenated organic materials requires high temperatures, i.e. 600.degree.-13000.degree. C., and requires long residence times to fully accomplish the complete destruction of the halogenated organic wastes (halocarbons).
Since the formation of the very problematic dioxins and dibenzofurans in incinerators leads to the formation of soot, halogen precursors, free-radicals, and the condensation phenomena, it is critical to remove the one essential element, the halogen, very early in the destruction process. In a steam-reforming detoxifier, no dioxins or debenzofurans are formed because of the absence of oxygen and flame-fronts. However in such a reactor, other aromatics, such as benzene, ethylbenzene, and the like, and polyaromatic hydrocarbons, such as naphthalene, pyrenes and the like are formed together with partially halogenated species. These aromatic compounds can lead to soot formation and particulate matter. Therefore, even in a steam-reforming process, the early removal of the halogen greatly improves the situation.